Optical liquid level sensor including built-in test circuitry

ABSTRACT

A liquid sensing apparatus for a fuel tank comprises a first optical sensor for producing an output having a first state that corresponds to the first optical sensor being exposed to air and a second state that corresponds to the first optical sensor being exposed to fuel; and a circuit for detecting the first and second optical sensor output states. The sensor circuit also has self test capability for testing both active and passive components. The sensor circuits can be accessed using a two wire only interface, and thermal stability can be achieved with the electronic circuit as shown.

Under title 35, United States Code, Section 119 (e), this applicationclaims the benefit of United States Provisional application bearing Ser.No. 60/013,352 filed Mar. 13, 1996.

Under title 35, United States Code, Section 119 (e), this applicationclaims the benefit of United States Provisional application bearing Ser.No. 60/013,352 filed Mar. 13, 1996.

BACKGROUND OF THE INVENTION

The invention relates generally to optical sensors for fluids. Moreparticularly, the invention relates to optical sensors of the type usedfor fluid level detection and that can discriminate different fluidtypes.

Optical fluid point level sensors are well known. Such sensors commonlyuse a prism having a conical tip. The prism is transparent to a beam ofinfrared and visible light launched into the prism from a light source(such as an LED). The infrared and visible light travels through theprism towards a surface of the conical tip and impinges on the surfaceat a determinable angle of incidence. The prism is made of a materialthat has a refractive index such that there exists a critical angle ofincidence at which all infrared and visible light is internallyreflected to another surface and back to a photodetector, such as aphototransistor. Whether such internal reflection occurs depends on therefractive index of the fluid to which the conical tip is exposed andthe angle of incidence. The critical angle is defined by the followingequation:

    θ.sub.c =sin.sup.-1 (n.sub.2 /n.sub.1)               Eq.1

where n₂ is the index of refraction of the fluid, and n₁ is the index ofrefraction of the prism conical tip. Thus, for air, n₂ =1.00 and forglass, n₁ =1.50. Accordingly, for total internal reflection the criticalangle with respect to air is about 42°. By comparison, if the conicaltip is exposed to water as the fluid, the refractive index of water is1.33. Thus the critical angle for total internal reflection with respectto water is about 62.5°.

By forming the conical surfaces such that the infrared and visible lighttransmitted therethrough is incident at 45°, the infrared and visiblelight will undergo total internal reflection (hereinafter "TIR") whenthe conical tip is exposed to air (because 45° is greater than thecritical angle of 42° for a glass/air interface), but will not undergoTIR when the conical tip is exposed to water (because 45° is less thanthe critical angle of 62.5° for a glass/water interface). By positioninga infrared and visible light detector to receive the infrared andvisible light that is internally reflected, the prism can be used as apoint level detector for the water level. The transmitted infrared andvisible light that is not internally reflected is refracted into thefluid, as is well known.

Note that for TIR to occur, the refractive index of the conical tip ishigher than the refractive index of all fluids which are to be detected(in this example, air and water).

Such a prismatic sensor can also be used to detect an aircraft fuel/airinterface when the prism material is made of a higher refractive indexsuch as 1.65, because the index of refraction for fuel is on the orderof 1.4 to 1.5. Thus, it is known to use such sensors for fuel leveldetection by detecting the ullage/fuel interface at different levels ina fuel tank.

A significant problem in aircraft fuel tanks, particularly largecommercial aircraft flying at high altitudes for extended periods oftime, is the accumulation of free water at the bottom of the tank. Thisfree water can adversely affect the performance of capacitance type fuelquantity sensors; although such erratic behavior can be used as awarning that water is accumulating in the tank. Typically, tank sumpsare opened to drain the water from the tanks. Most aircraft also havescavenge pumps that are used to mix the water with the fuel and burn itoff prior to buildup of any significant amount.

Free water is continually being generated in the fuel tanks. Duringascent, the fuel cools and water is thrown out of solution. Further,during descent, moist air is sucked into the tanks and condensationoccurs on the surface of the fuel and cooled structural members.

The fuel tanks on such aircraft can remain below 0° C. for several hoursafter landing. As a result, the free water freezes and the scavengepumps and sumps are ineffective. Even after refueling, the ice canremain for extended periods. Short layovers and improper fuel storageand fueling operations at remote locations can cause even more freewater to be loaded on-board the aircraft.

Conventional optic fluid level sensors such as just described areineffective in such circumstances because the prism/water interface doesnot cause TIR. Thus, water in the tank can be misinterpreted by suchsensors as being fuel.

Another significant drawback of known optic sensors is that theelectronics housed in the each sensor tend to be very sensitive tooperating temperatures and electromagnetic interference, thus requiringadditional circuitry for temperature compensation and filtering.

The objectives exist, therefore, for an optical sensor for fuel levelsensing that can discriminate between air, fuel and water. Such a sensorshould also be able to detect ice as well as liquid water, andpreferably should exhibit stable operation over a wide operatingtemperature range. Furthermore, such a sensor should use a minimalnumber of conductors for interfacing to control circuitry.

SUMMARY OF THE INVENTION

To the accomplishment of the aforementioned objectives, the presentinvention contemplates, in one embodiment, a liquid sensing apparatusfor a fuel tank comprising: a first optical means for producinganloutput having a first state that corresponds to the first opticalmeans being exposed to air and a second state that corresponds to thefirst optical means being exposed to fuel; second optical means forproducing an output having a first state that corresponds to the secondoptical means being exposed to air or water, and a second state thatcorresponds to the second optical means being exposed to fuel; andcircuit means for determining said first and second optical means outputstates.

The invention also contemplates an optic sensor that can discriminatebetween air and water in one mode, and fuel in another mode; oneembodiment of such a sensor being an optical detector for discriminatingbetween air or water and aircraft fuel comprising: a prism with afrusto-conical tip that receives infrared and visible light from a lightsource and internally reflects infrared and visible light to a infraredand visible light detector when said tip is exposed to air or water, andrefracts infrared and visible light into fuel when said tip is exposedto fuel, said prism comprising polyethersufone.

The invention also contemplates, in another embodiment, an electroniccircuit for interrogating an optical sensor, wherein such a circuitcomprises power source means for applying voltage to the infrared andvisible light source, and means for detecting the infrared and visiblelight receptor switch state based on current drawn from said powersource means, said electronic circuit being connected to the opticaldetector by not more than two wires.

The present invention also contemplates the methods of use embodied insuch apparatus and devices, as well as a method for liquid detection ina fuel tank, comprising the steps of: a) using a first prism tointernally reflect infrared and visible light from a infrared andvisible light source to a infrared and visible light detector when theprism is exposed to air, and refracting infrared and visible light awayfrom the detector when the prism is exposed to fuel; and b) using asecond prism to internally reflect infrared and visible light from ainfrared and visible light source to a infrared and visible lightdetector when the second prism is exposed to water, and refractinginfrared and visible light away from the detector when the second prismis exposed to fuel.

The present invention also comtemplates a two wire sensor configurationwith multi-function capabilities. The multi-function are activated, orcontrolled by the selection of the input voltage, and in turn, thefunction outputs are in the form of the current measured. One suchembodiment is for built-in-test. By activating the built-in-test, thesensor can be analyzed as to its current health.

These and other aspects and advantages of the present invention will bereadily understood and appreciated by those skilled in the art from thefollowing detailed description of the preferred embodiments with thebest mode contemplated for practicing the invention in view of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical schematic diagram of an optical point level fuelsensor and a circuit for interrogating such a sensor, in accordance withthe present invention;

FIGS. 2A and 2B are simplified schematic drawings of an air/fuel opticsensor suitable for use with the apparatus of FIG. 1;

FIG. 3 is a simplified schematic of a dual prism fuel level sensoraccording to the invention;

FIG. 4 is an enlarged more detailed view of a water/fuel discriminatoruseful with the dual prism apparatus of FIG. 3;

FIG. 5 is an electrical schematic diagram of a sensor circuit suitablefor use with the dual prism embodiment of FIG. 3;

FIG. 6 is another embodiment of a detector circuit suitable for use withthe present invention; and

FIG. 7 illustrates various exemplary operating conditions and loadcurrent levels for the embodiment of FIG. 5.

FIG. 8 is a simplified schematic block diagram of an optical point levelfuel sensor and a self testing circuit for interrogating such a sensor,in accordance with an alternate embodiment of the present invention.

FIG. 9 is a detailed electrical schematic diagram of an optical pointlevel fuel sensor and a self testing circuit for interrogating such asensor, in accordance with an alternate embodiment of the presentinvention.

FIG. 10a is a side view of an optical point level fuel sensor inaccordance with the present invention.

FIG. 10b is a top view of an optical point level fuel sensor inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, wherein like reference numeralsdesignate like or corresponding parts throughout different views, thereis shown in FIGS. 1, 2A and 2B, a first embodiment of an optic pointlevel sensing apparatus for fluid in a tank is generally designated withthe numeral 10. Such apparatus is particularly well suited for detectingfuel levels in an aircraft fuel tank. Although the invention isdescribed herein with particular reference to its use in aircraft fueltank applications, such description is intended to be exemplary andshould not be construed in a limiting sense. Those skilled in the artwill readily appreciate that the advantages and benefits of theinvention, as will be apparent from the following description, can berealized in other applications as well.

The apparatus 10 includes an optic sensor assembly 12, and a detectorcircuit 14 that is used to energize the sensor 12 and determine thesensor output conditions. In the embodiment of FIG. 1, all of thecomponents associated with the sensor 12 (as enclosed by the boxdesignated "sensor assembly") can conveniently be disposed in a singlehousing 16. In accordance with one aspect of the invention, it will benoted that only two wires or conductors 18,20 are needed to connect thecircuit 14 to the sensor 16 circuitry. As represented in FIG. 1, thesewires can be arranged as a twisted pair to reduce sensitivity toelectromagnetic interference (EMI) . The two wires 18,20 are used bothfor coupling electrical energy to the sensor 12, as well as to coupledata or signals corresponding to the sensor output back to the circuit14. Noise on the twisted pair can be suppressed by the use of EMIfilters 22, such as part no. 1270-016 available from Murata.

The sensor assembly 12 is disposed in a liquid tank, such as an aircraftfuel tank (not shown), such as by mounting the housing 16 on the bottomof the tank, for example. The detector circuit 14 can be remotelylocated with other electronic equipment of the aircraft, such as thefuel management electronics.

One embodiment of the optical elements of the sensor assembly 12 isshown in a simplified manner in FIGS. 2A and 2B. The sensor 12 includesa infrared and visible light source 24, such as an LED, and a infraredand visible light detector 26 such as a phototransistor. Attached to thehousing 16 (not shown in FIG. 2A, B) is a prism 28 that includes theaforementioned 45° conic tip 30. The sensor 12 in this case is designedto discriminate air and fuel so that the point level of the fuel/airinterface in a fuel tank can be determined. Collimating lenses can beused to improve the optical coupling between the photodevices 24, 26 andthe prism. The prism is preferably made of polyethersufone, availableunder the tradename RADEL A from Bronze and Plastics Specialties, thathas an index of refraction of about 1.65. This material has been foundto be well suited for fuel tank applications as it is compatible withaircraft fuel, and has a refractive index that is higher than therefractive index of the fuel, the latter typically being about 1.43. Thematerial has a transmissivity of about 73% at 930 nm, thus making itconvenient for use with standard LED and phototransistor devices.Polyethersufone is machinable and moldable to form the conical point 30in a conventional manner.

The optics portion of the sensor assembly 12 operates in astraightforward manner. FIGS. 2A and 2B illustrate the principlesinvolved for two conditions. In the condition represented in FIG. 2A,the conical point 30 is exposed to fuel (or stated more generally, to afluid having a refractive index higher than air but less than theprism). The LED 24 emits a beam of infrared and visible light 32 suchthat the beam is incident on a first surface 30a at an angle of about45°. Because the critical angle for the fuel/prism interface is greaterthan 45°, most of the infrared and visible light from the incident beam32 is refracted into the fuel. Preferably, the transmitted beam 32 has abeam width or spread (in FIG. 2A this angle is shown to be about 20°),some of the infrared and visible light incident on the first surface 30amay be reflected to a second surface 30b of the conical tip 30, at whichpoint it will then pass into the fuel. Essentially no infrared andvisible light, or very little infrared and visible light, returns to thephototransistor 26. In accordance with one aspect of the invention,using a transmitted infrared and visible light beam with a beam spreadsuch as 20°, in this case, reduces the sensitivity of the sensor 12 toresidual fuel droplets that might adhere to the conical surfaces. Otherbeam spreads, such as 10° for example, may also be suitable, dependingon the particular application. When the infrared and visible lightsource is an LED, the beam spread can be selected by the designer basedon the device selected, such as a device that includes a lens to providethe desired beam spread, as is well known to those skilled in the art.

The LED and phototransistor are arranged in the housing 16 such thatthere is good optical coupling between the photodevices and the prism.For example, the prism surface 30c that is adjacent the photodevices canbe polished, and the photodevices mounted near this surface.

In the case of FIG. 2B, the conical tip is exposed to air. Because thecritical angle for the air/prism interface is less than 45°, virtuallyall of the transmitted infrared and visible light beam 32 is internallyreflected back to the phototransistor 26 due to TIR, as representeddiagrammatically by the arrows in FIG. 2B. Therefore, the point level ofthe fuel surface 34 can be detected by monitoring the operation of thephototransistor 26.

With reference again to FIG. 1, from an electrical point of view, theLED 24 and phototransistor 26 are part of a sensor circuit 36 that canbe interrogated by the detector circuit 14. In accordance with antheraspect of the invention, the sensor circuit 36 can be accessed by remoteelectronics, such as the detector circuit 14, by a simple two wireconnection. This is accomplished, in general, by having the sensorcircuit 36 configured as a variable current load that varies as afunction of the on/off state of the phototransistor 26.

The wires 18,20 can be connected to the sensor circuit at input terminalnodes 38,40 respectively.

A noise suppression capacitor 42 is provided across the input nodes38,40 for enhanced noise immunity. The LED (D1) 24 is connected inseries with a first resistor R2 across the terminals 38,40. The R2resistor limits the current flow through the forward biased diode 24when the diode is emitting infrared and visible light. In this example,the value of R2 is selected such that the forward bias current throughthe diode 24 is about 7 ma (assuming a bias voltage across the inputnodes 38,40 of about 6 volts).

The phototransistor 26 is connected in series with a collector resistorR3 and an emitter resistor R4 between the input nodes 38, 40. A MOSFETtransistor 44 is configured as a switch to increase load current of thecircuit 36 when the phototransistor 26 turns on. The value of theresistor R4 is selected so that when the phototransistor 26 is off(corresponding to a wet state of the conical tip 30, as in FIG. 2A, forexample), the FET 44 gate voltage is below the gate-source thresholdvoltage, and the FET device is off.

When the phototransistor 26 turns on (corresponding to a dry state ofthe conical tip 30, as in FIG. 2B, for example), the phototransistorproduces an output current of about 300-400 μa. This current issufficient to produce a voltage across resistor R4 that exceeds thegate-source threshold voltage of the FET 44, thus turning the FET on.The FET turns on under these conditions and draws about 3 ma (for a 6volt bias voltage across the nodes 38,40), so that the total load of thesensor circuit increases to about 10 ma.

The 3 ma differential load current that is produced by the sensorcircuit 12 between the wet and dry states is detected by the detectorcircuit 14. The detector circuit 14 produces an output signal thatindicates the wet/dry state of the sensor assembly.

The detector circuit 14, in the embodiment of FIG. 1, includes aconstant voltage source 46, realized in the form of an operationalamplifier configured as a voltage follower in a known manner. A currentsense resistor 48 is connected in series between the amplifier output ofthe voltage source 46, and a node 49 that is connected to the positiveinput node 38 of the sensor circuit 36 by one of the wires 18 of thetwisted pair. The voltage at node 49 is a fixed reference voltage byoperation of the voltage source 46. A zener diode 50 is used to preventover voltage due to infrared and visible lightening strikes or shortcircuits.

The detector circuit also includes a comparator circuit 52. Thenon-inverting input (+) of the comparator 52 is connected to a resistordivider node 54 at the junction of two bias resistors 56,58. Theinverting (-) input to the comparator 52 is connected to the fixedvoltage side of the current sensing resistor 48. The resistors 56,58 areselected such that the comparator 52 threshold is nominally symmetricalabout the constant voltage source output at node 49. Small valuecapacitors (not shown) could be included at the comparator 52 inputs toreduce sensitivity to fuel sloshing or short electrical transients,however, their use is not required. Note that the detector circuit 14 asconfigured in FIG. 1 compensates for variations in the constant voltagesource output at node 38.

The circuit 14 includes two additional comparator circuits 120 and 122.The comparator 120 is used for an open circuit condition self-test, andthe comparator 122 is used for a short circuit condition self-test. Notethat the self-test comparators can be used with a detector circuit suchas shown and described with respect to FIG. 6.

The open circuit test is accomplished by appropriate biasing of thecomparator 120. As stated, the sensor circuit 36 has a minimum loadcurrent, in this case about 7 ma. Therefore, the comparator 120 isconfigured to monitor the voltage across the current sense resistor 48and to change state from high to low if the load current falls below adefined tolerance, for example, 5 ma. The tolerance can be set as afunction of the expected worst case load current condition for thephotodiode 24. An excessively low load current would be an indication ofa possible open circuit or similar fault in the twisted pair 18,20 orthe sensor circuit.

The short circuit test is accomplished by appropriate biasing of thecomparator 122. As stated, the sensor circuit 36 has a load current thatat its maximum can be defined by the current drawn by the photodiode 24and the switching FET 44. For the sensor circuit in FIG. 1, this loadcurrent maximum is about 10 ma. Therefore, the comparator 122 isconfigured to monitor the voltage across the current sense resistor 48and to change state from high to low if the load current exceeds adefined tolerance, for example, 15 ma. The tolerance can be set as afunction of the expected worst case load current condition for thephotodiode and MOSFET switch.

The detector circuit 14 further can include a series of logic gates 124configured as shown that logically combine the outputs of the variouscomparators 52, 120 and 122 to produce outputs that indicate theself-test functions and the sensor outputs.

When the sensor 12 is "wet", the load current of the sensor circuit 36is about 7 ma, as described, and in the "dry" state is about 10 ma. Thecurrent sense resistor 48 is selected such that, at a load current of 7ma (wet sensor), the output of the comparator 52 is logic low becausethe non-inverting input is biased below the inverting input; and whenthe load current is 10 ma (dry sensor), the output of the comparator 52is logic high because the non-inverting input voltage increases abovethe inverting input due to the effect of the increased load current.Thus, the detector circuit 14 provides an output signal (logic high andlow) that corresponds to the optical output of the sensor 12 (TIR and noTIR).

For the described embodiments, a suitable LED device is part no.SE5455-003 available from Honeywell, and a suitable phototransistor ispart no. SD5443-003 also available from Honeywell. A suitable device forthe MOSFET 44 is part no. JANTX2N6661 available from Motorola. Otherexemplary component values are provided in the schematics of the variousFigures herein, and it will be understood that these values will beselected depending on the particular application.

In accordance with another aspect of the invention, the sensor circuitdesign has inherent temperature stability. As is known, the photodevices24,26 exhibit considerable temperature sensitivity, and in particular,the LED power degrades as temperature increases, and the output currentof the phototransistor increases with temperature. However, the FET 44also is temperature sensitive, with the threshold voltage increasingwith temperature. The FET and phototransistor changes with temperaturetend to offset the possible degraded performance of the LED withtemperature. The net effect of all three devices changing withtemperature is that the performance of the sensor circuit is lessaffected by wide changes in operating temperature range.

The polyethersufone conical point detector and associated circuitry isthus useful for detecting a fuel/air interface, such as for a liquidpoint level sensor. However, because the refractive index of water isabout the same as for fuel, the 45° conical sensor design is not used todetect water in the fuel tank.

In order to discriminate water from fuel, the present inventioncontemplates, in another embodiment, a two prism sensor apparatus 60such as illustrated in FIGS. 3. The dual prism apparatus includes afirst optical sensor 12' that can be, for example, substantially thesame as the sensor 12 described hereinbefore. The sensor 12' is thusused to detect a fuel/air interface based on TIR occurring when thesensor conical tip 30' is dry, and refracting infrared and visible lightinto the fuel when the tip is wet. The sensor electronics (see FIG. 5)for the sensor 12' is substantially the same as the embodiment of FIG. 1(except for the load current value when the FET switch turns on).

The apparatus 60 further includes a second optical sensor 62. Exemplarydimensions are provided in FIG. 4 (as also in FIG. 2B). This opticalsensor can be used to discriminate water from fuel (and if desired, alsoto detect an air/fuel interface). As shown in more detail in FIG. 4, thesensor 62 is similar to the conical sensor 12', except that the tip 64is in the form of a truncated cone or frusto-conical contour. Thefrusto-conical tip thus includes three reflective surfaces 64a, 64b and64c, with the surface 64b being generally flat. An LED 66 andphototransistor 68 can be optically coupled to the truncated cone prism64 in a manner similar to the LED and phototransistor for the conicalsensor 12'. As shown in FIG. 3, the prisms can be disposed in a fueltank by means of a housing 70 that can be mounted to a tank wall. Thehousing 70 also conveniently encloses the sensor 60 electronics (FIG.5).

As shown in FIG. 3, for a top mounted apparatus 60, the air/fuel sensor12' is axially and sinfrared and visible lightly shorter than thewater/fuel sensor 62 so that it detects air before the second sensor 62.This is to avoid ambiguity at an air/fuel interface, because the secondsensor 62 will also detect air in the same manner as it detects water,i.e. TIR occurs if the truncated cone is exposed to air or water. Thus,the dual prism apparatus can be used to discriminate water/fuel/airinterfaces at the prism tips. If the apparatus 60 is bottom mounted, thesensor 12' would preferably be sinfrared and visible lightly longer thanthe sensor 62 to avoid the ambiguity.

The truncated cone prism 62 can also be formed from polyethersufone. Theconical surface is formed at an angle of 55° (with respect to thehorizontal axis as viewed in FIG. 4). This produces an angle ofincidence with respect to the center beam 72 of about 55°, which isgreater than the critical angle for the prism when exposed to water.Therefore, TIR will occur when the truncated cone is exposed to water(or air) and infrared and visible light will be reflected back to thephototransistor 68. When the truncated cone is exposed to fuel, all theinfrared and visible light essentially exits the prism 62 into the fuel.The flat surface 64b can be top cut for example with 400 grit wet/drypaper followed by 15 micron and 3 micron polishing disks. The conicalsurface 64a/c can be machined and vapor polished in a known manner. Foroptimum performance, the surface 64b should be parallel with the surface65. The angle formed at the juncture of the surface 64b and the conicalsurface 64a,c should also be very acute with minimal rounding.

In accordance with the invention, the sensor 62 not only detects liquidwater, but also will exhibit TIR when the water is frozen in the bottomof the fuel tank. The use of a wide beam of infrared and visible light,such as the 20° beam spread described hereinbefore, reduces sensitvityof the sensor 62 to ice accretion and residual fuel on the sensor tip64. If smooth ice forms on the surface, infrared and visible light isreflected off of the outer surface of the ice back to thephototransistor. However, rough ice formation is more common in a fueltank application and results in substantially less reflection. The widebeam spread helps reduce sensitivity to such ice formation. Thefrusto-conical sensor therefore, functions to accurately discriminatewater and ice from fuel.

Although the frusto-conical sensor 62 can distinguish water from fuel,it also will provide TIR for air. Therefore, the conical sensor 12' isused to determine whether the sensor 62 is exhibiting TIR due to wateror air.

With reference to FIG. 5, one embodiment of a sensor circuit 74 for thedual prism apparatus is shown. This circuit embodies similar features asthe circuit 36 of FIG. 1 in that it can be accessed using a two wireonly connection, and the sensors' status is detected as a function ofthe load current drawn by the circuit 74. Like components are identifiedwith the same reference number followed by a prime (').

The circuit 74 thus includes two terminal nodes 38' and 40'. The LEDs 66and 24' are connected in series with a current limiting resistor 76.When neither sensor 12', 62 exhibits TIR, the diodes 66, 24' draw about9.5 ma current (assuming a supply voltage of about 7.5 VDC).

The truncated conical sensor circuit 74a includes phototransistor 68connected in series with two biasing resistors 78,80 across the inputnodes 38',40' as shown. A first MOSFET 81 is provided with its gateconnected to the emitter of the phototransistor 68. The emitter biasresistor 80 is selected so that the FET 81 cannot turn on if thephototransistor 68 is off. A current limiting resistor 82 is seriesconnected between the drain and the positive node 38'. The source of theFET 81 is connected to the return node 40'.

The phototransistor 26' optical output is detected by a circuit 74b thatis substantially the same as the sensor circuit 36 in FIG. 1. Thus,phototransistor 26' is connected to a collector resistor 83 and anemitter resistor 84. The phototransistor emitter is also connected tothe gate of a second MOSFET 85. The MOSFET drain 85 is connected to thepositive input node 38' by a resistor 86.

Note that the conical sensor circuit 74b uses a 1 kohm drain resistor86, whereas the circuit 74a uses a 2 kohm resistor (as does thecorresponding circuit in FIG. 1). This is done so that the load currentchange caused by the second FET switch 85 turning on is twice the loadcurrent change caused by the first FET switch 81 turning on.

The circuits 74a and 74b operate in substantially the same manner as thecircuit 36 in FIG. 1, except for the actual load current values. Thus,the FET switches 81,85 function to provide a load current change thatrepresents the output state of the respective phototransistors 68, 26'.

A twisted wire pair 18', 20' can be used to connect the circuit 74 to adetector circuit illustrated in FIG. 6 and described hereinafter.

Operation of the circuit in FIG. 5 is such that four discrete loadcurrent levels are produced depending on the output states of theoptical sensors 12' and 62. The circuit 74 is a two terminal circuitrequiring only two wires for connection, and can be energized by adetector circuit of the configuration shown and described with respectto FIG. 6 herein. Accordingly, a voltage source is connected across theinput nodes 38' and 40', such as about 7.5 volts.

With reference to FIGS. 5 and 7, when both sensors 12' and 62 areexposed to air (as illustrated in FIG. 7A), both sensors exhibit TIR andcause maximum load current from the circuit 74. The FET switches 81, 85are both turned on so that the total load current is approximately thesum of the current drawn by the LEDs 24', 66 (about 9.5 ma), the currentdrawn by the first FET 81 (about 3 ma) and the current drawn by thesecond FET 85 (about 6 ma) . Thus, as noted in FIG. 7A the load currentfor both sensors exposed to air is about 18.6 ma.

When the conical sensor 12' is exposed to air but the frusto-conicalsensor 62 is exposed to fuel (FIG. 7B), the first FET switch 81 is offand the second FET switch 85 is on, thus producing a total load currentof about 15.6 ma.

When both sensors 12', 62 are exposed to fuel (FIG. 7C), both FETswitches 81, 85 are off so that the load current is at a minimum withthe LEDs 66, 24' turned on, or about 9.5 ma.

When the conical sensor 12' is exposed to fuel but the tip of thefrusto-conical sensor 62 is exposed to water (FIG. 7D), the first FETswitch 81 is on because the sensor 62 exhibits TIR. The second FETswitch 85 is off because water causes diffraction of infrared andvisible light through the sensor and prevents TIR. Thus, the total loadcurrent is about 12.6 ma.

When both sensors 12', 62 are exposed to water (FIG. 7E), the loadcurrent is also 12.6 ma.

Thus, the dual sensor arrangement of FIG. 5 provides four load currentlevels, each of which corresponds to a unique condition so that it canbe determined whether the sensors are exposed to air, fuel or water.

With reference now to FIG. 6, there is shown another embodiment for adetector circuit 90 that is useful to detect the discrete current loadsof the circuit 74. This detector circuit 90 detects the discrete loadcurrents of the sensor circuit 74 (FIG. 5) in a manner somewhat similarto detector circuit 14 of FIG. 1. Therefore, like reference numerals areused for like components followed by a prime (').

In accordance with the invention, the circuit 90 includes a constantvoltage source 46' which, in this case, is configured to produce a 7.5VDC fixed output at a reference node 91. This reference voltage isconnected to the input node 38' of the sensor assembly circuit 74 (FIG.5).

A load current sense resistor 48' is provided between the output 97 ofthe voltage source 46' amplifier and the reference node 91.

The reference voltage at node 91 is input to the non-inverting input (+)of an air/liquid comparator 92 and the inverting input (-) of afuel/water comparator 94.

The inverting input (-) of the comparator 92 is connected to a junctionnode 95 of a resistor divider network that includes a first resistor 96connected to the load current sensor resistor 48' at node 97 and asecond resistor 98 connected to the return. In this manner, theinverting input of the comparator 92 will vary as a function of the loadcurrent.

In a similar manner, the non-inverting input of the comparator 94 isconnected to a junction node 100 of another resistor divider networkthat includes a resistor 102 connected to the load current senseresistor node 97 and another resistor 104 connected to the return. Thus,the non-inverting input of the comparator 94 will vary as a function ofthe load current.

The resistor values for the divider network 96, 98 are selected so thatthe comparator 92 changes output state from high to low when the loadcurrent exceeds about 14 ma, which will occur, for example, when theconical sensor 12' is exposed to air but the frusto-conical sensor 62 isexposed to fuel. When the comparator 92 output goes low it turns on alamp 106 that indicates the conical sensor is exposed to air.

The resistor values for the divider network 102, 104 are selected sothat the comparator 94 changes output state from high to low when theload current is less than about 11 ma, which will occur, for example,when both sensors are exposed to fuel. Note that when the output of thecomparator 94 is low, a lamp 110 turns on to indicate the presence offuel.

The outputs of the comparators 92, 94 are logically combined by anothercomparator 108, which produces an output that indicates the presence ofwater. When the comparator 108 output is low, a WATER lamp 112 is turnedon.

Operation of the circuit of FIG. 6 will be best understood withreference to FIG. 7. When both optical sensors 12', 62 are exposed toair (FIG. 7A), the load circuit is about 18.6 ma so that the output ofcomparator 92 is low (AIR lamp 106 on) and the output of comparator 94is high (Fuel lamp 110 off) . Thus, the output of comparator 108 is alsohigh and the WATER lamp 112 is off.

When the conical sensor is exposed to air but the frusto-conical sensoris exposed to fuel (FIG. 7B), the load current is about 15.6 ma. In thiscase, the comparator 92 output is low (AIR lamp 106 on) and thecomparator 94 output is high (FUEL lamp 110 off). Thus, the output ofthe comparator 108 is high and the WATER lamp 112 is off.

When both sensors are exposed to fuel (FIG. 7C), the load current isabout 9.5 ma, and comparator 92 output is high (AIR lamp 106 off) andthe output comparator 94 is low (FUEL lamp 110 on). The output of thecomparator 108 will be high.

When the frusto-conical sensor 62 is exposed to water and the conicalsensor exposed to water or fuel (FIGS. 7D and 7E), the load current isabout 12.6 ma so that the output of the comparator 92 is high (AIR lamp106 off) and the output of the comparator 94 is high (FUEL lamp 110off). In this case, the output of the comparator 108 is low so that theWATER lamp 112 is on.

Although not shown in FIG. 6, the detector circuit 90 can be furtherprovided with self-test capability by the use of additional comparators,as previously described with respect to FIG. 1. For example, an opencircuit test could be realized by the use of a comparator arranged todetect when the load current falls below a minimum level, based on theexpected worst case current drawn by the LEDS. A short circuit testcould be realized by the use of another comparator that detects anexcessive load current, based on the worst case load current expectedwhen all LEDs and FET switches are on.

Those skilled in the art will further appreciate that the opticalsensors can also be used in a visual sensor mode. For example, the baseof the truncated cone can be visually monitored to detect for thepresence of water. Ambient infrared and visible light that impinges onthe base of the cone is reflected back to the viewer when the cone issurrounded by air or water and is dispersed when surrounded by fuel. Theinfrared and visible light at the base can either be natural infraredand visible light in dayinfrared and visible light hours or from asource such as a flashinfrared and visible light in darkness. Thedifference in infrared and visible light contrast between water and fuelas seen by the viewer is approximately 100 to 1. The base of the cone inthe presence of water appears as a donut with the sidewalls twice asbright as the flat top due to infrared and visible light being reflectedtwice off the frusto-conical surface. The cone could be screwed directlyinto the base or side of the tank. Commercial aircraft have an encloseddrain valve in the lowest part of the fuel tank. The cone can be mountedadjacent to the valve. The optimum prism for visual observation wouldhave an adjacent angle of 55 degrees for polyethersulfone and a topsurface diameter of 0.25 inches. The height would be determined by waterdepth to be sensed. The donut would encompass the complete sidewall inthe optimum design.

The sensor 12 and sensor circuit 14, for example, can also be used as adirect two terminal replacement for a thermistor bead fuel level sensor.The majority of fuel level sensors in use today are thermistor beadswhich are biased typically at about 50 milliamperes and in the dry statehave a surface temperature of about 175 degrees centigrade at anoperating environment of 70 degrees. The resistance of the thermistorbead is typically 90 ohms in the wet state and 60 ohms in the dry state.The circuits described herein for the interface electronics to theconical point sensor can simulate the thermistor bead and isinterchangeable with existing thermistor bead sensors.

The two terminal sensor interface is compatible with a number ofdetector interfaces. An important advantage of the optical sensor is theoperating temperature which is a maximum of 10 degrees centigrade abovethe operating environment. Thermistor beads have other problems besidesan operating temperature near the fuel flash point such as slow responsetimes, procurement difficulties in as much as they are customized forthe application in small production lots which makes them expensive, andlimited EMI levels.

Although the invention has been described with respect to the use of thedetector circuit in combination with two optical fluid level detectors(such as FIG. 5), those skilled in the art will readily appreciate thatany number of optical detectors can be used in combination with thedetector circuit. By simply using additional comparator circuits thatare configured to detect discrete load current changes, the two wireinterface can be used for a large number of optical fluid leveldetectors where each optical detector causes a detectable load currentchange based on its output condition. Thus, for example, a number ofoptical detectors could be used to detect fluid levels at differentlevels in a tank.

Referring now to FIG. 8, wherein an optical liquid level sensor system200 in accordance with an alternate embodiment of the present inventionincludes a variable state voltage source 210 connected via a pair oflines 211, 209 to an EMI and short circuit protection circuit 201. Theoutput of protection circuit 201 is provided on three lines 202, 203,204. Line 202 is the output voltage to a node 220 for energizing a 7volt switch circuit 205, an LED 232, a resistor 234 and a modulatorcircuit 206. Line 204 is a current return line. Line 203 is a controlline which is connected to a node 240 which controls 7 volt switchcircuit 205 and a 5 volt switch circuit 207. A resistor 282 is providedbetween circuits 205, 207. A node 284 connects between resistor 282 andthe cathode of LED 232. A resistor 290 is connected between node 284 andcurrent return line 204. A photo darlington transitor (PDX) 296 isconnected between resistor 234 and a node 301. Node 301 is connected toand controls modulator circuit 206 and is also connected to a resistor302. LED 232 and PDX 296 are provided within a frusto-conical sensor340, which is positioned so that the tip of the sensor protudes into afuel tank.

Sensor circuit 200 operates in three modes. These modes are 1 wet/drydetection mode; 2) a first BIT mode which forces the light path throughsensor 340, thereby imitating a dry condition; and 3) a second BIT modewhich disables the light path, thereby imitating a wet condition.Switching between modes is accomplished by simulating the sensor withthree distinct voltages from source 210. For example, the three voltagestates may be on the order of 4V, 6V, and 8V. Of course, other voltagesmay be utilized. The sensor state within each mode is determined bymeasuring the input current.

Electromagnetic interference (EMI) and electromagnetic pulse (EMP)protection is provided by circuit 201 from causing an incorrect reading,and to prevent large energy transfer into the fuel tank during lightningstrikes.

Normal wet/dry detection is accomplished by setting V_(in) toapproximately 4 volts. The 7 volt circuit 205 and 5 volt circuit 207 arenot activated at 4 volts and therefore have no effect on the circuits.Resistor 290 is chosen sot that approximately 0.5 ma flows through LED232, which produces a suitable amount of energy to ensure that only adry condition on cone sensor 340 will result in sufficient energytransfer to PDX 296 to turn it on. The PDX on state will cause the totalsensor current increase, thereby allowing external detection anddiscrimination between wet and dry conditions.

The first BIT mode is intitiated by setting V_(in) to approximately 6volts, which turns on 5 volt circuit 207, thereby increasing the LEDcurrent draw to approximately 5 ma. With 5 ma going through LED 232,there is sufficient energy reflected in cone sensor 340 to always turnon PDX 296. Therefore, the sensor circuit 200 current will always behigh and within a given range. A failure in any of the 5 volt switchcircuit 207, 7 volt switch circuit 205, LED 232, PDX 296 or currentmodulator circuit 206 will cause the sensor circuit 200 current to falloutside the expected current range.

The second BIT mode is intitiated by setting V_(in) to approximately 8volts, which turns on 7 volt circuit 207, thereby shorting out all LED232 current. With 0 ma going through LED 232, there is insufficientenergy reflected in cone sensor 340 to ever turn on PDX 296. Therefore,the sensor circuit 200 current will always be low and within a givenrange. A failure in any of the 5 volt switch circuit 207, 7 volt switchcircuit 205, LED 232, PDX 296 or current modulator circuit 206 willcause the sensor circuit 200 current to fall outside the expectedcurrent range.

The sensor circuit 200 is failed if either of the BIT modes fails.

Referring now to FIG. 9, wherein an optical liquid level sensor system200 in accordance with an alternate embodiment of the present inventionincludes a variable state voltage source 210 connected via a line 211 toa fuse 212 (F1) which is thereafter connected to resistors 214 (R1), 216(R2), and a capacitor 218 (C1). A node (or line) 220 connects R1, acapacitor 222 (C2), a resistor 224 (R6), the emitter 226 of a transistor228 (Q4), the anode 230 of a LED 232, a resistor 234 (R13), and resistor236 (R16). A node 240 connects R2, a capacitor 242 (C3), the cathode 244of a 6.8 volt Zener diode 246 (Z1), a resistor 248 (R7), and the emitter250 of transistor 252 (Q2). The anode 256 of Z1 is connected to aresistor 254 (R3). R3 is thereafter connected to the base 258 of atransistor 260 (Q1) and a resistor 262 (R4). R7 is connected to the base262 of transistor 252 (Q2) and the cathode 264 of a 4.3 volt Zener diode266 (Z2). A resistor 268 (R8) is connected to the anode 270 of Z2. Thecollector 272 of Q2 is connected to a resistor 274 (R9) which isconnected to the base 276 of a transistor 278 (Q3) and a resistor 280(R10). A resistor 282 (R11) is connected between a node 284 and thecollector 286 of Q3. Node 284 connects the collector 286 of Q4, thecathode 288 of LED 232, and a resistor 290 (R12). R13 is connected to anode 292 which connects the collector 294 of a Photo DarlingtonTransistor 296 (PDX) and a capacitor 298 (C4). The collector 300 of PDX296 is connected to a resistor 302 (R14) and a resistor 304 (R17). R17is connected to the base 306 of a transistor 308 (Q5). The collector 310of Q5 is connected to resistor 236 (R16) and the collector 312 of atransistor 314 (Q6) . The emitter 316 of Q5 is connected to the base 318of Q6 and a resistor 320 (R15). It can be seen that Q5 and Q6 arearranged in a Darlington Pair.

Operation of the circuit 200 is as follows. Voltage source 210 provides3 voltage states, preferably 4 volts, 6 volts, and 8 volts. LED 232 andPDX 296 are provided within a frusto-conical sensor 340,which ispositioned so that the tip of the sensor protudes into a fuel tank.Detection of the presence of fluid in the tank (i.e. normal operation)is done at the low voltage level of voltage source 210. Resistors R1, R6and R12 are picked so that at the low voltage level, approximately 0.5mA is conducted through R12. At that current level, the LED will bebright enough to reflect enough energy to turn PDX on in a dry condition(i.e. the tip of sensor 340 is not immersed in fluid). A wet conditionwill cause infrared and visible light to be absorbed into the fluid,which causes PDX to turn off, thereby keeping transistors Q5 and Q6 off.When Q5 and Q6 are off, circuit 200 will draw little current from thevoltage source 210 which, as described hereinbefore, is interpreted asbeing a wet condition. In a dry condition, TIR is produced within cone340, thereby turning on PDX, Q5 and Q6, thereby causing a high currentdraw condition out of voltage source 210. The states may be created bymodulating either the LED output or the photo sensor gain. During dryconditions the LED energy is fully reflected, thereby producing a largeoutput from the photo sensor (photo diode, photo transistor or photoDarlington). During wet conditions most of the energy escapes into theliquid. However a small amount of energy is reflected from the flat onthe end of the cone thus the photo sensor receives less energy, but itis measurable and discernable from when the LED is turned off. Thedifference between the low current draw and high current draw conditionstherefore represents wet or dry data by a detection circuit not shownherein, but further described hereinbefore in the previous figures.

When voltage source 210 is raised the middle range voltage, forinstance, 6 volts, a much greater current flows through R12, forinstance on the order of 5 mA. Under these conditions, LED 232 providesso much infrared and visible light to the conical sensor 340 internalreflections therein continuously turn on the PDX regardless of whethercone 340 is emersed in fluid or not. Raising the voltage to the middlevoltage range therefore exercises the circuit in order to determinewhether or not the LED circuit and the PDX circuit are working properly.

Raising the voltage source output to the highest level, for instance 8volts, causes Z1 to conduct, thereby turning on Q1 which turns on Q4,thereby shorting out LED 232. Under these conditions, no infrared andvisible light is reflected in the cone 340 and PDX remains offregardless of whether cone 340 is emersed in fluid or not. The highestvoltage range from the state voltage source therefore is a self test ofwhether the circuit turns off under the appropriate conditions.

Preferred approximate values for the components illustrated n FIG. 9 areprovided below in TABLE I.

                  TABLE I    ______________________________________    R1, R2             47 ohms, .1w, 5%    R3, R8             13.3k ohm, .1w, 1    R6, R10, R15       47k ohm, .1w, 5%    R5                 36k ohm, .1w, 5%    R11                1.2k ohms, .25w, 5    R4, R7, R9         22k ohm, .1w, 5%    R12                6.2K ohm, .1w, 5%    R13                1k ohms, .1w, 5%    R14                43k ohm, .1w, 5%    R16                1k ohms, .25w, 1%    R17                7.5k ohm, .1w, 5%    C1                 1000 pf, 100v    C2, C3             .01 uf, 100v    C4                 100 pf, 100v    Q1, Q3, Q5, Q6     NPN 2N2222AUB    Q2, Q4             PNP 2N2907AUB    CR1                6.8v 054087 CDI    CR2                6.8v 054086 CDI    CR3                054076-4 OPTEK    Q7                 054077 OPTEK    F1                 Fuse, .062A                       Littlefuse    ______________________________________

The detection circuit illustrated in FIGS. 8 and 9 therefore provides atwo wire sensor configuration with multi-function capabilities. Themulti-function are activated, or controlled by the selection of theinput voltage, and in turn, the function outputs are in the form of thecurrent measured. One such embodiment is for built-in-test. Byactivating the built-in-test, the sensor can be analyzed as to itscurrent health. This is particularly important when the sensor is usedin an environment which is almost always in one state (wet or dry). Inthis case, a failure, if gone undetected, may never show up until thealternate state is present, and some undesirable effect results (i.e.filling overflow, or run out of fuel).

Table II hereafter provides a logic chart for the circuit of FIG. 9.There are three operable states. In the LED off state, the LED circuitsensor is always off. In the LED on (high gain) state, the LED saturatesthe cone thereby turning PDX always on. In the LED normal gain state,PDX is turned on in dry contitions and turned of in wet conditions.

                  TABLE II    ______________________________________                 Wet/     Light Sensor                                    Fault    States       Dry      Detection Detection    ______________________________________    Led Off      Wet      0         1                 Dry      0         1    Led on High  Wet      1         0    Gain         Dry      1         0    Led On Normal                 Wet      0         NA    Gain         Dry      1    ______________________________________

Referring now to FIGS. 10a and 10b, wherein an optical point level fuelsensor 400 in accordance with the present invention includes anapproximately cylindrical body section 410, a frustoconical section 412and a flat Dreflective tip 414. The cone is preferably made frompolyethersulfone. Height A is preferably on the order of 0.25 inches,height B is preferably on the order of 0.40 inches and angle C ispreferably on the order of 50 degrees. The diameter of section 410 ispreferably on the order of 0.50 inches. The surfaces of sections 412,414 should be vapor polished by annealing the parts in an oven at roomtemperature then raising the temperature to 380 degrees Farenheit,holding there for four hours and then allowing the oven to cool to roomtemperature again.

While the invention has been shown and described with respect tospecific embodiments thereof, this is for the purpose of illustrationrather than limitation, and other variations and modifications of thespecific embodiments herein shown and described will be apparent tothose skilled in the art within the intended spirit and scope of theinvention as set forth in the appended claims.

We claim:
 1. A liquid sensing apparatus for a liquid tank with built intest capabilities comprising: an optical means for producing an outputhaving first and second states; and circuit means for operating saidoptical means in a selected one of three modes, a first mode under whichsaid optical means produces said first and second output states whenexposed to air and liquid, respectively; a second mode that is a testmode under which said optical means produces a fixed first output statewhich changes to said second output state only upon a fault in eithersaid circuit means or said optical means; and, a third mode that isanother test mode under which said optical means produces a fixed secondoutput state which changes to said first output state only upon a faultin either said circuit means or said optical means.
 2. The apparatus ofclaim 1 wherein the liquid tank is an aircraft fuel tank.
 3. Theapparatus of claim 1 wherein said circuit means comprises means fordiscriminating said first and second output states with a two wireelectrical connection between a power source, said optical means andsaid discriminating means.
 4. The apparatus of claim 3 wherein saidcircuit means determines said output states based on said power sourceoutput current supplied to said discriminating means.
 5. The apparatusaccording to claim 1 wherein said optical means comprises a prism, alight source and a light detector that receives light from said lightsource reflected from said prism to produce the first output state whenthe prism is exposed to air, said prism refracting light from the lightsource to the liquid when said prism is exposed to the liquid so thatthe light detector produces the second output state.
 6. The apparatus ofclaim 5 wherein said prism is disposed in a fuel tank so as to detect anair/fuel interface.
 7. The apparatus of claim 5 wherein said prism haseither a conical contour or a truncated conical contour.
 8. Theapparatus of claim 7 wherein said prisms comprises polyethersulfone. 9.The apparatus of claim 5 wherein the circuit means includes a powersource controllable to supply power to the circuit means and the opticalmeans at first, second and third voltage potential levels correspondingto the first, second and third selectable modes.
 10. The apparatus ofclaim 9 wherein the circuit means includes a first switch that isenergized to increase the light output of the light source to effect thesecond mode of the optical means when the power source is controlled tosupply power at the second voltage potential level.
 11. The apparatus ofclaim 10 wherein the circuit means includes a second switch that isenergized to decrease the light output of the light source to effect thethird mode of the optical means when the power is controlled to supplypower at the third voltage potential level.
 12. The apparatus of claim11 wherein the second switch, when energized, decreases the light outputof the light source to substantially zero.